The present invention relates to a device for simultaneously improving the performance of cabinets, windows, counters, chests, stalls, or units for displaying and selling foodstuffs that are conserved at a temperature lower than ambient temperature. Refrigerated or deep-freeze cabinets are in very widespread use in stores for dispensing foodstuffs, in pharmaceutical outlets, or more generally whenever it is necessary to conserve goods at a temperature lower than ambient. In the text below, refrigerated or deep-freeze cabinets, windows, counters, chests, stalls, or units are referred to generically as xe2x80x9ccold sales cabinetsxe2x80x9d.
In the present state of the art, refrigerated cabinets, windows, or counters, and more particularly so-called xe2x80x9copenxe2x80x9d cabinets, all present the feature of operating by virtue of curtains of refrigerated air subjected to forced circulation along the open face. Said open face can be horizontal or vertical. Whatever the precautions taken to guide such curtains of air, it is found that some mixing takes place between the refrigerated air and the air of the premises in which said cabinets are placed. A consequence of this partial mixing is that a significant quantity of water vapor is contained in the flow of air driven by the fans and put into thermal contact with the evaporator of a cold sales cabinet. This water vapor is inevitably trapped at said evaporator where it becomes deposited as a progressive buildup of ice. This buildup of ice has the effect of progressively constituting an insulating sheath around the evaporator and its fins, thereby reducing the heat exchange coefficient between said element and the circulating air, and also increasing head losses, thereby significantly reducing the flow rate of refrigerated air.
After cold sales cabinets have been in operation for a few tens of minutes (which period is a function of the humidity of the ambient air, the dew point temperature, and the evaporation temperature of the refrigerating fluid), a significant drop is observed in the efficiency of the curtain of air, and consequently a rise is observed in the temperature of the inside of the cabinet and of the foodstuffs it contains.
To combat that phenomenon of frost appearing and the associated losses of performance, various solutions have been proposed which have turned out to be relatively unsatisfactory.
Firstly, it has been proposed to warm up the evaporator periodically, either by stopping the circulation of refrigerating fluid within said component, or by providing it with heater cords or electric resistances, or indeed by heating the air (WO 98/06987 to Hislop, FR-2 610 708 to Bosch Siemens, EP-0 768 053 to Sanyo, EP-0 403 459 to Electrolux).
Proposals have also been made to duplicate the evaporator, with one of the elements being defrosted while the other one is in operation (WO 94/26154 to Hussmann).
Proposals have also been made to dehumidify the air prior to passing over it over the evaporator (application No. 91/09664 - 2 679 988 to A.R.M.I.N.E.S.).
Proposals have also been made to build up a store of low temperature, using a large mass of ice or a eutectic for the purpose of taking over from the evaporator for the length of time it requires to be defrosted (application No. 88/02314 - 2 611 383 to Toshiba).
Finally, numerous dispositions and combinations of single, double, or triple air curtains have been proposed in which said air curtains are either heated or cooled for the dual purposes of limiting uptake of water vapor in the flow of air and limiting condensation on the evaporator (WO 97/17003 to Bonnet Neve, EP-0 709 046 to Sanyo, FR-2 615 082 to Bonnet Refrigeration, WO 94/23620 to Northampton Refrigeration).
Proposals have also been made to defrost the evaporator by using jets of superheated gas or steam.
None of those solutions turns out to be genuinely satisfactory, whether from the point of view of the energy consumption of cold sales cabinets or the genuine performance of such apparatuses and their ability to keep foodstuffs under temperature conditions that comply fully with the legislation in force.
In terms of cost, it is found that the need to combat frosting phenomena on evaporators leads to evaporators, fans, and air circuits being over-dimensioned. The length and the frequency of the essential defrosting cycles in presently-available apparatuses are such that these pieces of equipment are used for their refrigeration function during less than 80% of their total operating time. The remainder of the time they are used for a function that is the exact opposite from that for which they are designed, i.e. to be subjected to or to cause heating which temporarily and partially cancels their effects. Experience shows that presently-manufactured and sold cold sales cabinets are subjected to large temperature variations over time, which makes it very difficult to comply with the standards in force; these variations are particularly large between the beginning and the end of the defrosting period, which period inevitably gives rise, in the present state of the art, to significant heating of the air curtain and thus of the foodstuffs. Experience also shows that in the present state of the art and regardless of the particular solution implemented to combat evaporator frosting phenomena, presently-manufactured and sold cold sales cabinets cannot provide a temperature that is uniform throughout their inside volume: the phenomenon of evaporator frosting does not take place identically on all parts of said element, so the resulting head losses are necessarily non-uniform, and as a result the temperature and the flow rate of the air curtain(s) are themselves non-uniform. This state of affairs leads to certain inside portions of the cabinet being at a temperature that is far from its nominal temperature, and that is prejudicial to good conservation of foodstuffs.
It is also observed in the present state of the art that evaporators and elements situated close to evaporators in cold sales cabinets can be the subject of microbial or bacterial proliferation phenomena, with pockets of air or collections of water warmed by the action of the defrosting means presently in use providing media that encourage the development of germs, bacteria, or microbes. Since, by construction, these evaporators and their immediate vicinity are difficult to access with any kind of cleaning or decontamination means, it is unfortunately observed that these organisms can become transferred to the foodstuffs or goods conserved inside cold sales cabinets, with the vector being air which is circulated by the fans.
That said, non-published experiments performed by the authors of the present application concerning cold sales cabinets fitted with defrosting devices included in the prior art have shown that the formation of frost on the outside surfaces of evaporators does not have a totally negative effect on the performance of such cabinets: it is found that initially the appearance of a thin film of frost on the outside surface of an evaporator has the effect of increasing its real heat exchange area and thus of increasing heat exchange efficiency to a significant extent; this applies in particular when the layer of frost covering the outside surfaces of the evaporator is of a thickness that does not exceed 1 millimeter (mm). After a few minutes of operation, this positive effect is overtaken by the negative effect of the insulation caused by depositing a layer of frost and/or ice having a thickness of several millimeters; thereafter, this layer of frost and/or ice creates an obstruction, and then a complete blockage between the fins of the evaporator, which ends up by preventing all air circulation and thus preventing any heat exchange.
The invention proposes means for taking advantage of the positive effects of the appearance of frost on the surface of the evaporator and for eliminating the negative effects of said phenomenon.
More generally, the invention proposes making cold sales cabinets that mitigate all of the drawbacks described above.
An object of the invention is to provide a cold sales cabinet capable of ensuring that all of the foodstuffs it contains has a temperature that is constant in time and uniform in space.
Another object of the invention is to propose a cold sales cabinet having better energy efficiency, i.e. consuming less energy for a given level of performance.
An additional object of the invention is to make it possible to provide a cold sales cabinet of a cost price that is significantly reduced, which result is obtained by optimizing elements such as the evaporator and the fans which can operate in regular manner for up to 100% of the time, and as a result can be smaller in size than those presently used in cabinets seeking to provide performance at the same level as that provided by a cold sales cabinet of the invention.
Another additional object of the invention is to propose a cold sales cabinet having means that are of low cost and effective for eliminating any proliferation of bacteria, germs, or microbes on the evaporator and in its immediate environment, and consequently avoiding any risk of contaminating foodstuffs or goods conserved inside the cold sales cabinet.
It is known that defrosting can be performed, i.e. molecules of water in solid form can be melted to form molecules of water in liquid form, by using energy which is dissipated in the form of microwaves. For example, document GB-2 278 668 discloses a refrigerator in which the evaporator is defrosted by microwave type radiation. The evaporator and the microwave source are placed in a compartment of the refrigerator which is separated from the food by a screen which prevents the microwaves from reaching the food. The water which results from melting the frost formed on the evaporator is collected in a trough placed in the compartment.
That is the closest document although it does not relate to a cold cabinet of the type of the invention since it does not have means for circulating a flow of air. In the present invention, air circulation is very important or even essential in that it is circulating air which conveys low temperature and causes accelerated formation of frost on the evaporator.
The present invention seeks to provide a cold cabinet with forced air circulation in which the defrosting of the evaporator by microwaves gives rise neither to a rise in the temperature of the air curtain nor to a large amount of energy being consumed.
To reduce energy consumption, the present invention provides means for continuously removing water from the compartment containing microwaves. By removing the water, the quantity of energy required for defrosting purposes is reduced, since the microwaves no longer serve to heat that water and are therefore constrained to cause the frost on the evaporator to melt directly. Microwaves have a tendency to heat liquid water first and to heat solid water only subsequently. By removing the liquid water from the compartment or enclosure as it is formed, the microwaves do not have the chance of dissipating therein, and as a consequence there is a saving in radiation and thus in energy, giving rise to operation that is less expensive and faster.
If the water is not removed immediately as it forms, microwave defrosting means are found to be practically useless in terms of melting the frost that forms on the fins.
In another embodiment, the removal means consist in the circulation means. The forced air flow takes away a fraction of the liquid water present on the evaporator, removing it from the enclosure by transporting it in the air.
In a variant or in combination, the removal means comprise at least one orifice formed in the bottom wall of the enclosure. The water flows along the fins of the evaporator and falls under gravity. It is then collected in a metal receptacle and removed to outside the enclosure. Water preferably passes through the bottom wall of the enclosure subjected to the radiation via one or more orifices, a grid, or a perforated sheath, with the open section being selected as a function of the nominal frequency of the generator so as to constitute a wave trap. This passage can be accelerated by slopes converging on the evacuation orifices, where such slopes are conferred on said bottom wall by construction.
The evaporator, or at least its active portion, is contained in a microwave-proof enclosure so that the microwaves are reflected on the walls of said enclosure and release their energy firstly to the ice or frost that they cause to melt. A waveguide, preferably fitted with deflectors, serves to direct the waves onto the fins of the evaporator. It is also possible to use wave stirring means for directing or distributing the waves into preferred zones or directions. In a preferred embodiment, at least two of the walls are made in the form of perforated sheets of grids having openings of dimensions that are large enough to allow a transverse flow of air to pass through the enclosure and small enough to prevent microwave radiation escaping out from the enclosure. In an alternative embodiment of cabinets of the invention, removable partitions are installed which can be solid or partially perforated, said partitions being automatically retracted or folded away during the short periods in which the microwave generator(s) is/are in operation.
In the preferred embodiment, it will be understood that it is appropriate to make the waveguides in such a manner that the waves are directed exclusively or at least preferentially towards the evaporator, or indeed towards the zone of said evaporator which is the most subject to frosting and icing phenomena, i.e. the zone where the forced air comes into contact with said element. It is also possible to use surface waveguides placed on the outside surfaces of all or some of the fins of the evaporator. These surface waveguides are coupled to the microwave generator and made of a dielectric material whose dielectric constant is close to that of frost and whose thermal conductivity is close to that of copper; these two characteristics are advantageous: the first makes it possible to obtain good propagation of waves between the surface waveguides and the layer of frost while the second makes it possible to ensure that the surface waveguides do not act as insulators reducing the heat exchange efficiency between the evaporator and the flow of air. To obtain both of these characteristics simultaneously in spite of them being, a priori, contradictory, the surface waveguides are made of a material belonging to the family of polymers or resins doped by including particles of non-magnetic metal.
In any event, the spacing between the fins constituting the heat exchange surface of the evaporator is selected in such a manner that said spacing optimizes the passage of microwaves between two adjacent fins, i.e. the spacing should be greater than 5 mm, and in such a manner that said spacing makes it possible to obtain a large heat exchange area between the evaporator and the air, i.e. a spacing of less than 10 mm. In preferred manner, the evaporator fitted to a cold sales cabinet of the invention is thus provided with fins at a spacing lying in the range 5 mm to 10 mm.
The evaporator can be provided with a plurality of long fins projecting significantly beyond the alignment constituted by the ends of standard fins. Under such circumstances, the spacing between two long fins should be selected so that said spacing enables good microwave penetration between a long fin and its neighbors of the same category (i.e. more than 5 mm), while the spacing between standard fins is selected in such a manner that said spacing (selected to be less than 5 mm) is not compatible or is poorly compatible with microwaves penetrating into the gaps between the standard fins.